The present invention relates to a method and apparatus for diagnosing a malfunction in a clutch actuator of a motor vehicle. The clutch actuator receives an input command signal of variable magnitude from a control unit that may belong to an automatically shifting transmission, an automated clutch or a continuously variable transmission. The input command signal controls the position of the clutch actuator and thereby sets the state of engagement of the clutch. As a common trait of the inventive method as well as existing methods, the clutch-actuator position or state of engagement of the clutch is measured and an output signal corresponding to the measured actuator position is made available.
In the search for safety-monitoring concepts for automatically shifting transmissions, it has been found that many critical situations arise mainly in connection with the actuation of the clutch, e.g., if the clutch moves into or out of engagement at times when this is not intended. It is therefore of the highest importance to have real-time information about the current status of the clutch.
In connection with automatic or automated clutches or transmissions, it is a known concept to provide a measurement signal that indicates the position of the clutch actuator. However, as a matter of principle, the possibility of failure has to be taken into account for a position-sensor of the clutch actuator just as for any other technical component. Thus, the position of the clutch actuator as indicated by the sensor is not unconditionally reliable. Consequently, in connection with monitoring the safety of automated shift transmissions, it is highly advisable to include the clutch position or clutch actuator position in the monitoring process.
As an attempt to satisfy this need, a known concept of monitoring the position sensor of the clutch actuator involves a plausibility test based on minimum and maximum signal values. However, this allows only certain extreme malfunctions of the sensor to be discovered, e.g., a short circuit or a break in the cable.